That's Admiral Ackbar. Ackbar was the leader of the Rebel
military in the final battle scene of "Return of the Jedi." He's
a relatively minor character with very little screen time, but
he's most famous for belting "It's a trap!" when the Rebels
realize the Empire lured them into a trap.

Here's the scene:

It's also a scene that has spawned meme
after meme on the internet over the years.

There's a small chance this is just another alien that's the same
species as Ackbar, but
an earlier leak appears to show the character in the movie.
There's a very good chance we'll get to see Ackbar again when
"The Force Awakens" comes out on December 18.